Rent Refund Entitlement?

[SpacemanR]

I moved into my rented flat almost 4 months ago. On the move in day I was shown flood damage in the bathroom which was not there when I first viewed the flat 2 months previously. The flood damage consists of a hole in the ceiling directly above the toilet from which dirt, dust and small chunks of masonry fall onto the toilet or whoever is using it. The ceiling is also damp, smells bad and is a breeding ground for small black flies.
Despite assurances from the letting agent that progress was being made it is only in the last two weeks that a surveyor has inspected the damage and even now it looks as if the insurance company is stalling on paying out the cost of the repair estimate.
In my Lease the landlord claims that structural repairs will be carried out within a reasonable period of time. My own personal opinion is that a reasonable period of time would have been at most 4-6 weeks.
Considering the above am I entitled to a partial rent refund? Any help would be appreciated.

Thanks.

R